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article imageVideo: Supersonic Ping-Pong gun fires a ball faster than sound

By JohnThomas Didymus     Feb 6, 2013 in Technology
A team of US researchers at Purdue University, Indiana have built a ping-pong gun that can fire balls at supersonic speeds of more than 400 meters per second or Mach 1.2. The gun fires a ping-pong ball faster than a supersonic F-16 jet.
According to Purdue University technology blog, Mark French, associate professor of mechanical engineering technology and two of his Ph.D students have created a supersonic ping-pong gun that can shoot a ping-pong ball at Mach 1.2 (about 915 miles per hour).
Ordinary ping-pong guns, of course, do not fire balls at supersonic speeds. Designing one that can fire balls at supersonic speeds requires some modifications that involve the use of a de Laval nozzle designed in 1888 by a Swedish engineer for steam turbines. The de Laval nozzle is also used in supersonic jet engines.
Purdue University technology blog says the ping-pong gun device has helped French and his students, Craig Zehrung and Jim Stratton, to demonstrate how a de Laval nozzle (also called a convergent-divergent nozzle) works to convert subsonic gas flow into supersonic flow.
Super-sonic ball blows a neat hole in a ping-pong paddle
Super-sonic ball blows a neat hole in a ping-pong paddle
Mark French et al.
The research scientists said "the ping-pong gun is a popular and compelling tool for demonstrating acceleration and the power of pressure differentials in air. It has even proven to be a challenging problem for computational fluid dynamics since it encompasses a range of aerodynamic phenomena that are particularly difficult to model. The design of the team's ping-pong gun is reassuringly low-tech. It consists of a pressure chamber sealed with several layers of duct tape."
Gas characteristic along a de Laval nozzle
Gas characteristic along a de Laval nozzle
MIT Technology Review reports that the supersonic ping-pong gun consists of a pressure chamber sealed with two or three layers of Duck Tape. When the pressure in the chamber exceeds about 620 kPa, the tapes burst releasing a pressure wave into the de Laval nozzle (see diagram below). The pressure wave is accelerated in the de Laval nozzle and moves into the barrel of gun where it propels a ping-pong ball.
Diagram of supersonic ping-pong ball showing de Laval nozzle
Diagram of supersonic ping-pong ball showing de Laval nozzle
Mark French et al.
The design of the nozzle is critical for generating supersonic propulsion forces. The nozzle is pinched in the middle, creating an overall asymmetric hourglass shape.
As the air pressure wave enters the nozzle it accelerates under compression as it flows toward the pinch. The compressed air flows to the pinch where it attains to the speed of sound. The pressure is accelerated to supersonic speed as the air flows away from the pinch and the tube expands.
The supersonic flow passes into the gun barrel where it is used to propel the ping-pong ball to supersonic speeds
The team said: "We measured the velocity of the ball using a high-speed video camera set to record at 16,000 frames/second with an exposure time of 1/128,000 second. The speed of the ball was measured through the use [of a] graduated calibration grid with 0.25 inch (0.635cm) spaces. Velocity variation between shots was relatively small, a few per cent. Video results from a typical shot gave a velocity of 406.4 m/sec, which corresponds to Mach 1.23."
The researchers warned: "It is important that the device be used only in a controlled environment. The increase in velocity greatly increases the kinetic energy of the ball, so care must be taken to ensure that the ball is captured after launch. Additionally, it is very important to keep all observers behind the muzzle and safe from any bounces or ricochet."
According to Purdue tech blog, French told NBC News: “You can shoot Ping-Pong balls through pop cans and it is great, it is so captivating, it is so compelling that you can get kids’ attention and once you’ve got their attention, you can teach them something."
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